The present invention relates to a system for a shockless downshift control to engine braking.
An automatic transmission is known which includes a frictional device, namely an overrunning clutch, which is to be engaged to establish an engine brake running status. This automatic transmission has two engine brake running ranges. During forward drive with the fourth gear, if a driver presses a power button, a 4-3 downshift is effected before the overrunning clutch is engaged to produce engine braking, while, if the driver sets a manual valve to 2 range position, a 4-2 downshift is effected before the overrunning clutch is engaged. Engine brake running status is not established until the overrunning clutch is engaged. The inertia energy to be absorbed by the overrunning clutch differs from one state after a 4-3 downshift has been made to another state after a 4-2 downshift has been made. Thus, the level of working hydraulic pressure to be applied to the overrunning clutch has to be set to a relatively low level after effecting the 4-3 downshift, but to a relatively high level after effecting the 4-2 downshift. In order to accomplish this purpose, a pressure reduction valve, namely an overrunning clutch pressure reduction valve is fluidly disposed between a shift valve and the overrunning clutch. A so-called "2" range pressure as high as the line pressure is supplied to the overrunning clutch when the manual valve is set at the 2 range position. This causes the level of the hydraulic pressure to the high level.
An object of the present invention is to improve a downshift control to engine brake running range such that a level of a working hydraulic pressure applied to a frictional device to be engaged for engine braking is tailored to the interia energy to be absorbed by the frictional device.
A specific object of the present invention is to provide a system for controlling a downshift to engine brake running range wherein the level of working hydraulic pressure applied to the frictional device for engine braking is also variable with vehicle speed since the inertia energy to be absorbed by the frictional device is variable with the vehicle speed.
A still further object of the present invention is to provide a system for controlling a downshift to engine brake running range wherein without any addition of hardware, the level of working hydraulic pressure applied to the frictional device for engine brake running range is tailored to the inertia energy to be absorbed by the frictional device.
Another object of the present invention is to provide a system for controlling a downshift to engine brake running range wherein a downshift is quickly effected prior to engagement of the frictional device for engine braking to improve engine braking quality.